This memorandum briefly discusses the physical metallurgy, heat treatment, mechanical properties, stress-corrosion properties, and fabrication of the 9Ni-4Co-XC steels. This family of steels was developed specifically to meet the requirements of the aircraft and aerospace industries for materials possessing high strength and high toughness. By varying the composition and heat treatment, the 9Ni-4Co-xC steels can develop strengths in the 200 to 300 ksi tensile-strength range and reportedly possess high toughness. The lower carbon grades have excellent weldability, developing strength in the 180 to 200 ksi range with high toughness without the need for preheat or postweld-heat treatment. (Author).

Recent studies of the developments in welding steels with yield strengths greater than 150 ksi have included low-alloy martensitic steels, medium-alloy martensitic steels, nickel maraging steels, and bainitic steels. Only weldments from medium-alloy martensitic steels and nickel maraging steels have mechanical properties approaching those of the base plate without a complete postweld heat treatment. The most serious problem with the other steel is low toughness in the weld fusion zone. Adequate weld metal toughness under conditions of elastic strain can be obtarined over the entire 150 to 225 ksi yield-strength range only if the tungsten-arc welding process is used. Processes with higher deposition rates can produce comparable weld deposits only in the lower portion of the range. Above a yield strength of 200 ksi, 18Ni maraging steel weldments have the best combination of strength and toughness. Below 200 ksi, the HP 9-4-25 medium-alloy martensitic steel and 12Ni maraging steel weldments have nearly equal properties.